In fashioning sintered zirconia ceramics for use as insulating refractory and catalyst supports, it is desirable to develop a zirconia material with a porous microstructure with stable porosity at the high use temperatures (&gt;1400.degree. C.).
Processes now utilized to create porosity in materials used in such applications result in products that all have limited use at high temperatures, either being unable to withstand thermal shock or decrease substantially in porosity at high temperature use.
Underfiring, firing at temperatures such that sintering is incomplete, is the simplest way to achieve porous ceramics. Subsequent use of such materials even at temperatures below the firing temperatures usually result in continued sintering reduction of total open porosity.
Other methods utilized to create porous ceramics, such as the use of burnout mediums and foaming, not only adversely affect the rheological and elastic properties of the batch thereby complicating the forming processes such as extrusion, pressing, slip casting, etc., but also cause fire cracking as a result of the entrapment of carbonaceous matter or trapped gases causing localized heating and thermal stresses during firing or at high use temperatures.
Other adverse conditions incident to the use of the foaming and burn out techniques is the evolvement of polluting smoke and carbon monoxide caused by the combustion of the trapped burn out materials, or of foaming agents and their combustion products.
The sintering of zirconia as discussed in Industrial Ceramics by Felix Singer et al., pgs. 1141-1142, published 1963 by Chapman & Hall Ltd. London, mentions, as a means of preparing the sintered zirconia, the use of zirconium hydroxide as a binder when extrusion or pressing was utilized in the process. The use of metal hydroxides as a binder was disclosed in U.S. Pat. No. 1,694,924, which specifies maintaining a low amount of the bonding substance, viz. 1 wt.% of the finished refractory article. German Pat. No. 519,796 discloses similar plasticizing additives or binders to be added to zirconium oxide batches as those disclosed in Singer et al. (supra) in an amount of from 1% to 10%, but does not specifically disclose zirconium hydroxide as one such additive. It is generally recognized in the art that a minimum addition of binder or plasticizing agents to refractory ceramic batches is preferred in order to maintain the mechanical, thermal and chemical strengths of the objects produced from such sintered bodies of U.S. Pat. No. 1,694,924 and German Pat. No. 519,796.
The preparation of compositions of fibrous zirconia in a matrix of microporous zirconia, as patented in U.S. Pat. No. 3,736,160, discusses the use of a liquid zirconium compound and a refractory powder. The zirconia cement used in the application employed was a mixture of a liquid containing a zirconium compound preferable in an aqueous solution and a refractory powder.